Effective permittivity and permeability of coated metal powders at microwave frequency

Recent developments of the microwave heating of compacted metal powders bring a demand on calculation of corresponding effective optical parameters. Since the existing models cannot be applied to such powders, in this paper new effective dielectric permittivity and magnetic permeability are introduced by means of the combination of Mie theory and Bruggeman's effective medium. It is demonstrated that both an insulating shell and the skin effect should be taken into account. The shell drastically modifies the effective permittivity, while ignoring the skin effect brings to the permittivity an error as large as 30% at 30 GHz for copper powder with grains of 50μm radius. The induced currents are shown to affect the magnetic properties of the powders. The strong skin effect substantially reduces the permeability of magnetic grains, and thus the intrinsic permeability measured in experiments can be much less than the real one. The strong dependence of the effective parameters on the microstructure of powders suggests a variety of possible heating scenarios in experiments.